Electro-hydraulic servo-valves are known in a large variety. Such servo-valves comprise most of the time two stages and sometimes 3 stages. Generally, the first or pilot stage comprises a torque motor which cooperates with a jet flapper plate amplifier. The pilot stage provides for an amplification of the pilot oil flow which is proportional to an input signal. The pilot oil flow will then cause a movement of the spool in a second or main stage. The main stage, in turn, will provide for additional amplification of the pilot oil flow and supplies a working oil flow which is proportional to the pilot oil flow and, consequently, also proportional to the input signal. The actual value of this working oil flow is generally indirectly determined by means of a mechanical characteristic, like for example the movement of the spool. This actual value is then supplied in the form of an electrical signal to a control apparatus. The just described known electro-hydraulic servo-valve requires a pilot stage with the result that a good sensitivity is obtained. As a consequence, however, there is always a high flow of hydraulic pilot oil, which causes losses.
Also known are servo-valves using an electro-motor which directly actuates the spool of a valve, in particular, the spool of a directional control valve. For example, the shaft of said electro-motor is provided with a crank- or excenter means for direct actuation of the spool. If in such a servo-valve the valve spool gets stuck (stuck condition), the electro-motor has to be designed such that the force supplied by the electro-motor is sufficient to remove the spool out of such a stuck position (or stuck condition). Frequently, the stuck condition occurs because a chip of material is trapped between the control edges of the valve. So as to release the spool from the stuck condition the chip has to be severed, so that the valve spool can resume its movement. The force which is required for this purpose and which has to be generated by the electro-motor can be called the "chip shear force", because it will shear or sever the chip. So as to make sure that such a motor can supply the required chip shear force it has to be very large. This is generally disadvantageous and, moreover, the motor will become sluggish because of its size.
With regard to specific prior art, attention is drawn to German patents 22 32 566 and 32 47 953, as well as German laid-open applications 31 37 419 and 32 46 298.
It has to be emphasized that the high required shear force requires a large motor which due to its large mass will allow only for low acceleration. However, high acceleration is desirable, so as to provide for a fast response.
It is an object of the present invention to overcome the disadvantages of the prior art.
It is a another object of the invention to provide an electro-hydraulic servo-valve such that an optimal dynamic response as well as the best possible frequency characteristic is obtained.
According to another object of the present invention, the servo-valve should have a small size, a low weight, and a reliable function.
In accordance with a further object of the invention, the servo-valve as well as the control motor allow for a direct actuation of the valve spool.